I'm NOT a KJV Onlyist. However, here's an alleged problem with modern translations that I encountered on Facebook by a KJV Onlyist:
I don't claim to know which is the correct translation. However, I don't think the modern translations are necessarily wrong, for the following reasons (which I posted on Facebook).
"Prosper" might be legitimate translation in light of the next verse (6). The context implies that he's boasting that he's secure and that calamity won't strike him. Similar to other passages in the OT where the wicked boast in their riches and earthly blessings.
The tabernacles of robbers prosper, and they that provoke God are secure; into whose hand God bringeth abundantly.- Job 12:6 KJV
They spend their days in wealth, and in a moment go down to the grave.- Job 21:13 KJV
Rest in the LORD, and wait patiently for him: fret not thyself because of him who prospereth in his way, because of the man who bringeth wicked devices to pass.- Ps. 37:7 KJV
For I was envious at the foolish, when I saw the prosperity of the wicked.- Ps. 73:3 KJV
Righteous art thou, O LORD, when I plead with thee: yet let me talk with thee of thy judgments: Wherefore doth the way of the wicked prosper? wherefore are all they happy that deal very treacherously?- Jer. 12:1 KJV
The NET Bible footnote on this verse says, "Heb “they are firm, his ways, at every time.” The verb חַיִל (khayil, “be firm, be strong”) occurs only here and in Job 20:21, where it has the sense “endure.”"
John Gill in his commentary says the following (notice Jarchi's interpretation):
To God and to his people; or, "his ways cause terror" (a), so Aben Ezra; make men fear; as antichrist has made the whole world tremble at him, Rev_13:4; or, "his ways are defiled", as the Septuagint and Vulgate Latin render it; for to him is nothing pure, his mind and conscience being defiled, Tit_1:15; or, "his ways always remain" (b); they are always the same, there is no change in them for the better: or they "prosper" (c) as Jarchi interprets it; and this is sometimes stumbling to the saints, Jer_12:1;
Albert Barnes prefers the interpretation of the KJV of the word. But Barnes nevertheless admits that it's a difficult word to translate. Here's what Barnes says:
Long QUOTE:
Psalms 10:5
His ways are always grievous - His paths; his manner of life; his conduct toward God; his dealings with men. The word rendered “are grievious,” יחילוּ yāchiylû - has been variously rendered. The Latin Vulgate renders it, “His ways are defiled.” So the Septuagint. Coverdale renders it, “His ways are always filthy.” Prof. Alexander, “His ways are firm.” So DeWette, “Es gelingen seine Wege.” Horsley, “His ways are confident.” This variety in the interpretation arises from the ambiguity of the original word - חול chûl. The meaning of this word, as given by Genesius, is to turn round, to twist, to whirl; and hence:
(1) to dance;
(2) to be whirled, or twisted upon anything;
(3) to twist oneself with pain, or to be in pain;
(4) to bear or bring forth;
(5) to tremble, to quake;
(6) to be strong or stable, as things twisted are.
Hence, he translates this passage, “his ways are firm, or stable, that is, all his affairs prosper.” But it seems to me plain that this is not the idea in the mind of the psalmist. He is not dwelling on the prosperity of the wicked, or on the result of his conduct, but on his character. In the previous verses he had stated some of the traits in his character, and the subsequent verses continue the description; hence, it is natural that we should expect to find some special feature of his character referred to here, and not that there should be an allusion to the stability of his affairs. It seems to me, therefore, that the exact idea here is, that his ways, or his modes of feelling and conduct were always perverse and forced, and hard; that there was always something tortuous and unnatural about him; that he was not straightforward and honest; that he did not see things as they are, and did not act in a plain and upright manner.
End QUOTE
When it comes to interpreting the Psalms, I always profit from reading Joseph Addison Alexander's commentary (various versions freely online at archive.org).
Here's a Screen Shot of of Alexander's commentary on this verse.
Click on picture to better read the comments.
I could have written more, but that should suffice to show that the modern translations do not commit a "gross" error.